1. Field of the Invention
The present invention relates to an optical-transmission apparatus and, more particularly, to an optical-amplifying apparatus used to amplify optical signals.
2. Description of the Related Art
Generally, optical-amplifying apparatuses are used to amplify the intensity of input optical signals and to transmit the amplified optical signals over a long distance with minimum errors. Such optical-amplifying apparatuses have important operation characteristics, such as gain, noise figure, and saturation output power. As erbium doped fiber amplifiers are known to have high gain, low noise figure, and large saturation output power, they have been widely utilized in many backbone networks or metro networks. However, the erbium doped fiber amplifiers have disadvantages in that they are expensive, and have relatively large sizes and limited amplification bands.
In contrast, semiconductor optical amplifiers are capable of solving the short comings of the erbium doped fiber amplifiers in that their costs are low, their sizes are large, and their amplification bands can be relatively and easily adjusted. However, as the semiconductor optical amplifiers have high noise figures, their actual application is inevitably limited.
The conventional semiconductor optical amplifiers tend to be small sizes and their amplification bands can be changed by adjusting the composition of gain materials. Moreover, they can be manufactured at a low cost.
In the case of general gain-clamped semiconductor optical amplifiers, they have good gain precedence and saturation output power. However, as they have a very high noise figure of more than 8 dB, they have many limitations when they are used in transmission systems. In the case of Raman amplifiers, they have very low noise figures. However, since they have a very low-light amplification efficiency, they require high power laser diodes in order to obtain enough gain.
In order to solve these disadvantages, a new optical-amplifying apparatus technology has been recently proposed, which has a structure that combines a Raman amplifier and a semiconductor optical amplifier. In such a combined structure, an optical signal obtains Raman gain before being inputted to the semiconductor optical amplifier, so that effective gain is increased. In addition, a signal-to-noise ratio (SNR) of the semiconductor optical amplifier can be improved by the Raman gain, so that the effective noise figure is reduced.
FIG. 1 shows a conventional optical-amplifying apparatus. As shown, the optical-amplifying apparatus includes a single mode fiber (SMF) 110, a Raman optical amplifier constructed with a wavelength selective coupler (WSC) 130 and a laser diode (LD) 120, and a semiconductor optical amplifier (SOA) 140.
In operation, the laser diode 120 outputs a pumping light S2 having a wavelength of 1400 nm to 1500 nm. The wavelength selective coupler 130 has a first port connected to the single mode fiber 110, a second port connected to the semiconductor optical amplifier 140, and a third port connected to the laser diode 120. Thus, the wavelength selective coupler 130 provides the single mode fiber 110 with the pumping light S2 inputted to the third port, and outputs a Raman-amplified optical signal S1 inputted to the first port to the second port.
The single mode fiber 110 is pumped by the pumping light S2, and then Raman-amplifies and outputs the optical signal S1 inputted via a stimulated Raman-scattering effect. The semiconductor optical amplifier 140 amplifies and outputs the Raman-amplified optical signal S1.
However, the conventional optical-amplifying apparatus as described above has a hybrid structure, defined by the semiconductor optical amplifier and the Raman optical amplifier, and integrates an expensive laser diode with the semiconductor optical amplifier, thereby increasing the material cost and the manufacturing cost. In addition, a separate wavelength selective coupler is required in order to integrate two devices with each other, and optical loss occurring in the connection portion between the devices deteriorates the optical amplification characteristic. Furthermore, a larger space is required in order to install the combined devices.